Various etchants are well known for etching molybdenum and tungsten. For example, molybdenum may be easily etched in ferricyanide solutions as disclosed in Acocella et al., U.S. Pat. No. 4,747,907, the disclosure of which is incorporated by reference herein. Similarly, tungsten may be etched by a mixture of hydrogen peroxide and aqueous ammonia. This tungsten etchant, however, has a shelf life of only about 8 hours. Tungsten etches very slowly, if at all, in ferricyanide solutions.
For many purposes, the performances of these etchants are acceptable. These etchants, however, are not satisfactory in some applications.
To illustrate this, the etching of molybdenum is typically effected in a strongly alkaline solution of ferricyanide: ##EQU1##
The pH of such an etchant solution is typically maintained between about 12.0 and 13.0 and temperature at about 50-55 degrees Centigrade.
David and Kurdziel, "Ozone Reoxidation of a Ferricyanide Bath for Etching Molybdenum," Metal Finishing, p. 47-49 (May 1988), have deduced the rate law for ferricyanide etching of molybdenum. It was concluded that by enhancing a ferricyanide solution with molybdate and by buffering the solution to control pH, good etching results may be obtained. It was necessary to maintain the pH at a relatively high level, 12.0 to 12.5.
One such application where this etchant would be unsatisfactory is when the molybdenum, for example, is incorporated in a laminate structure which is comprised of the metal and a base-sensitive low dielectric constant polymer such as a polyimide. These types of polymers will hydrolyze in strong bases and thus cannot survive current methods of etching molybdenum.
It would thus be desirable to have an etchant for molybdenum, and tungsten as well, which has a lower pH that will not adversely affect base-sensitive materials.
Lowering the pH of a molybdenum etchant, however, is not a trivial process. If the pH of a ferricyanide etch bath drops below about 10-11, the molybdenum blackens due to the formation of polymolybdates as follows: ##EQU2##
This stops the etching process completely.
Thus, a low pH etching process for molybdenum must preserve the etching rate and suppress the formation of undesirable molybdate polymer residues, as well as not hydrolyze companion polymers in the laminate or composite structure.
It has been discovered, however, that the pH may in fact be lowered in a molybdenum etchant solution without slowing down the etching rate and while suppressing the formation of undesirable molybdate polymer residues. It has also been discovered that the pH may be lowered for a tungsten etchant solution as well. The key is in the formation of heteropoly compounds, a class of compounds known in the literature for many years. See, for example, Tsigdinos, "Heteropoly Compounds of Tungsten and Molybdenum" in Topics In Current Chemistry, 76 (Springer-Verlag, 1978), the disclosure of which is incorporated by reference herein.
Accordingly, it is a prime object of the present invention to have a near neutral pH etching solution for effectively etching molybdenum and tungsten.
It is a further object of the invention to have a near neutral pH etching solution that is capable of being recycled.